There are known a CD and a DVD as an optical disc, and an information recording and reproduction apparatus (drive apparatus) capable of reading and writing information from and on both of them is spread. Wavelengths of laser beams used for recording and reproduction are different between the CD and the DVD. Therefore, as the above-mentioned drive apparatus, there is known one which uses an optical pickup of a two-wavelength compatibility type, equipped with a two-wavelength laser capable of emitting laser beams having a laser beam wavelength for the CD (substantially 780 nm, hereinafter also referred to as “infrared wavelength”) and a laser beam wavelength for the DVD (substantially 660 nm, hereinafter also referred to as “red wavelength”). Generally, the two-wavelength laser is a package equipped with two-wavelength laser diodes (LDs).
On the optical pickup using the laser of the two wavelengths (i.e., a red wavelength and an infrared wavelength), a physical position of the LD of each wavelength on the optical pickup is different. Therefore, on the above-mentioned optical pickup, there is used an optical element for correcting a space between the two LDs in order to receive a returned light from the optical disc by a single light-detecting element. The optical element is formed by a diffraction element (hologram element), and has a function to make optical axes of the two-wavelength laser beams coincident with each other.
Optical pickups, using the two-wavelength lasers and having a function to adjust the optical axes of the returned light from the optical disc on an optical detector, are disclosed in Patent References-1 and 2, for example. The Patent Reference-2 discloses an element correcting a color aberration generated in a going path of the laser beam from a light source up to the recording medium and a positional shift of a laser emitting point, when the two-wavelength laser is used as the light source.
However, there is recently proposed a so-called Blu-ray disc (BD) as an optical disc using a much shorter laser beam (substantially 405 nm, hereinafter also referred to as “blue wavelength”). Therefore, the above-mentioned optical pickup of the two-wavelength compatibility type cannot be used in order to realize the compatible drive apparatus capable of reading and writing the information from and on the BD in addition to the CD and the DVD. Namely, an optical pickup of a three-wavelength compatibility type becomes necessary.
In addition, since a numerical aperture (NA) of the BD is larger than numerical apertures of the CD and the DVD, a large spherical aberration is problematically generated due to a thickness error of a cover layer of the disc. Therefore, in reproducing the BD, it may become necessary to provide a spherical aberration detecting function on the optical pickup in order to detect and correct the spherical aberration. An example of the optical pickup having the spherical aberration detecting function is disclosed in Patent Reference-3.
Patent Reference-1: Japanese Patent Application Laid-open under No. 2003-177226
Patent Reference-2: Japanese Patent Application Laid-open under No. 2002-237081
Patent Reference-3: Japanese Patent Application Laid-open under No. 2003-45048